Field of the Invention
The invention pertains to the field of processing materials using microwave energy. More particularly the invention pertains to apparatus and methods for generating precise levels of microwave power across a broad frequency range.
Description of Related Art
The use of microwave power for industrial processes, including heating, is well established and practiced on a large scale, particularly for applications such as drying, where the principle material to be acted upon is water. It has also more recently been applied to the curing of thermoset polymers and adhesives. It is well known that the use of a single-frequency microwave source in a multimode cavity can lead to non-uniformities related to the establishment of standing waves within the cavity. To solve this problem, Variable Frequency Microwave (VFM) systems were developed, as taught, for example, in U.S. Pat. Nos. 5,521,360; 5,721,286; and 5,961,871.
In general, VFM systems have a multimode applicator cavity and a microwave power supply capable of broadband operation; here, broadband is defined as the capability to sweep the applied frequency over some useful range about a chosen center frequency. Model calculations taught in U.S. Pat. No. 5,961,871 show the relationship between bandwidth, cavity size, and center frequency on the overall spatial uniformity of microwave energy in the cavity 18. The block diagram, FIG. 1, shows the prior art for a control system that includes a microwave power supply that typically has a microwave frequency source, comprising a voltage-tunable YIG oscillator and a voltage-controlled attenuator, whereby signals from a control system are applied to produce a small-signal microwave output of a given frequency, bandwidth, waveform, or other selected characteristics. This microwave signal is then passed to a high power microwave amplifier, typically a TWT, to produce a high power signal having the aforementioned selected characteristics. A control system monitors the forward and reverse microwave output power. A second control loop monitors the process using temperature, as shown, or another suitable process parameter and provides closed loop feedback to adjust the level of microwave power automatically.
As shown in FIG. 1:
Small signal components 10 include: a YIG oscillator 11, which generates microwave frequencies at low power; and a voltage controlled attenuator 12, which adjusts small signal RF power level. A fixed attenuator (not shown) limits maximum signal strength to a safe level, and an RF Bandpass Filter 13 ensures RF signal is in band for high power amplifier.High Power Components include: a High Power Amplifier 14, which generates high power output from small signal input; and an Isolator 15, which prevents reflected power from the curing cavity reaching the high power amplifier.Measurement Components include: a Directional coupler 16, which provides a highly attenuated RF signal useful for RF power measurement. Crystal Detectors 19 convert the low level RF signal from the Directional Coupler to a useful DC control voltage 20a, 20b. 
In addition the prior art control system contains software based self calibration procedures to account for multiple systemic nonlinearities, e.g., the variation of TWT gain across the working bandwidth, FIG. 2. The microwave uniformity obtained with the aforementioned calibration routine, combined with closed-loop process control produce a level of microwave power output that has proven suitable for applications such as curing adhesives, curing polymers, processing ceramics in various applications, particularly those in electronics manufacturing.
However, in the development of processes for high value products such as thermal processing of semiconductor wafers, including coatings on wafers, it was found that the prior art variable or swept frequency control systems were unable to produce the precise level of control needed, and furthermore, that the calibration and setup procedures were cumbersome and time-consuming.
Objects and Advantages
Objects of the invention include: providing an improved broadband microwave source for heating and other purposes; providing a method for controlled microwave heating; providing a method for broadband microwave processing capable of open-loop operation; providing an improved method for processing semiconductor wafers; providing a VFM control system that is more robust and stable under varying environmental conditions; and providing a VFM control system that is easier to set up and calibrate.
These and other objects and advantages of the invention will become apparent from consideration of the following specification, read in conjunction with the drawings.